The present invention relates to a speech/voiceband data discriminator for determining whether a given digital input signal is a speech signal or a voiceband data signal.
With a view to effective utilization of long-distance lines for satellite communication and submarine cable communication, high efficiency digital circuit multiplication equipment (DCME) combining high efficiency speech coding (for instance ADPCM) and digital speech insertion techniques has been developed. This equipment requires different bit rates for the encoding of speech signals and that of voiceband data signals. For example, while speech signals are encoded at 32 kbps ADPCM, encoding of voiceband data signals, especially high speed voiceband data signals, at this bit rate would invite a data error rate higher than the permissible limit. In order to keep the data error ratio within the permissible limit, voice-band data signals should be encoded at a higher rate than speech signals are, for instance at 40 kbps ADPCM.
Therefore, the aforementioned high efficiency DCME requires a speech/voiceband data discriminator in order to distinguish between speech and voiceband data signals among time division multiplexed signals and lead them to their respective encoding circuits.
Such a speech/voiceband data discriminator is disclosed, for example, by S. Casale et al. in IEEE GLOBECOM'88 (Global Telecommunications Conference & Exhibition, 1988), Conference Record Vol. III, pp. 1419-1427. As illustrated in FIG. 2 of this article, the speech/voiceband data discriminator according to the prior art computes short time energy and zero crossing count for every one of three different signals--a given signal, a high frequency component resulting from the passing of this original signal by a high pass filter and a low frequency resulting from the passing of the original signal by a low pass filter--and the zero crossing counts of differential signals, and decides whether the given signal is a speech signal or a voiceband data signal by comparing the ranges of distribution of the computed results with the corresponding thresholds. Each of the thresholds, which this decision refers to, is set on the basis of the statistical analysis of sampled data collected in advance for speech and voiceband data signals. The final discrimination is made as to whether the input signal is a voice signal or a voiceband data signal with reference to the individual results of decision.
The conventional speech/voiceband data discriminator referred to above is so composed that the computed data of short time energy and zero crossing count are entered into separate decision circuits, whose decision results are further entered into a final decision circuit. Since the decision results of each decision circuit are denominated in one or the other of two levels, i.e. either "0" or "1", much of the information indicating correlations between short time energy and zero crossing count is lost. Accordingly, there can be obtained no sufficient quantity of information required for accurate discrimination between speech and voiceband data signals on various lines differing from each other in signal-to-noise ratio, resulting in a low discrimination ratio relative to the quantity of data processed.
Moreover, a vast volume of experimental data and a vast number of analyzing manhours are required to so set the threshold of each decision circuit as to permit the final decision to be made without inconsistency from the decision results of individual decision circuits.